PHYLOGENETIC ANALYSIS OF LYMNAEID SNAILS BASED ON 18S RDNA SEQUENCES

The 18S rDNA sequences of the six most common European Lymnaeidae spec ies (Mollusca: Gastropoda: Basommatophora) have been obtained by direc t PCR cycle sequencing and silver staining methods. The sequence align ment and secondary structures of the 18S rRNA gene of Lymnaea stagnali s, L. auricularia, L. peregra, L. palustris, L. glabra, and L. truncat ula are analyzed. This gene proves to be a good marker for both specif ic determination and supraspecific lymnaeid phylogeny. The malacologic al importance is evident, considering the specific determination probl ems of individual snails and the present systematic chaos in Lymnaeida e due to their pronounced morphoanatomic uniformity, which makes a cla ssification by traditional methods impossible. The majority (17) of th e total of 43 nucleotide-substituted positions appears to be confined to a small region included in helix E10-1 of the variable region V2, e nabling species group distinction: (1) the first sequence is common to L. auricularia and L. peregra; (2) the second sequence is unique to L . truncatula; and (3) the third sequence is identical for L. glabra, L . palustris, and L. stagnalis. The other 26 nucleotide-substituted pos itions are dispersed over the entire gene, although four grouped nucle otide positions in helix 6 of V1 are of interest in distinguishing L. glabra from both L. palustris and L. stagnalis. The phylogenetic trees obtained by comparison with four other molluscan species (a polyplaco phoran, two bivalves, and a stylommatophoran gastropod) show the prese nce of four well-defined subgenera among the genus Lymnaea sensu Info: (1) Lymnaea (Radix), (2) Lymnaea (Galba), (3) Lymnaea (Leptolimnaea), and (4) Lymnaea (Lymnaea). Two branches, L. auricularia-L. peregra-L. truncatula and L. glabra-l. palustris-L. stagnalis, are worth mention ing from the parasitological point of view, since the two digenean spe cies of large medical and veterinary impact transmitted by lymnaeids, Fasciola hepatica and F. gigantica, appear to be linked to the first b ranch.